The present invention relates generally to improvements in or relating to the control of ion dose in an ion implantation process, and more particularly to a method for compensating for the total ion dose implanted into semiconductor wafers by an ion implantation system during the implantation process.
Ion implantation uses charged particles (ions) to penetrate beneath a material""s surface, which gives the material unique electronic, mechanical, or chemical properties. Ion implantation is deemed as a key technique in the microelectronics industry, It is also used in other manufacturing sectors for its demonstrated potential for hardening of surfaces and for enhancing the corrosion properties of metals. Within the semiconductor manufacturing industry, ion implantation techniques are used to introduce impurity atoms into semiconductor substrates to alter the conductivity of the substrate in a controlled fashion. In ion implantation, electrically charged ions are accelerated under the action of an electric field and implanted into a solid target, i.e., a semiconductor wafer. Ion implanters generally comprise an ion beam generator for generating a beam of ions, a mass analyzer for selecting a particular species of ions in the ion beam, and a means to direct the ion beam through a vacuum chamber onto a target substrate supported on a holder.
The cross-sectional area of an ion beam is dependant upon various factors such as beam line configuration, degree of focusing applied to the ion beam, the gas pressure along the beam line, the energy of the ion beam, and the mass of the ions. Typically, the cross-sectional area of the ion beam at the target is less than the surface area of the target, e.g., a sen-conductor substrate. This requires beam traversal over the substrate in a one- or two-dimensional scan such that the beam covers the whole surface of the substrate. There are several two-dimensional scanning techniques commonly used in ion implantation systems to effect beam traversal: electrostatic and/or magnetic deflection of the ion beam in relation to a fixed target; mechanical scanning of the target substrate in two dimensions relative to a fixed ion beam; and an amalgamation of both techniques involving magnetic or electrostatic deflection of the ion beam in one direction and mechanical scanning of the target substrate in another, usually orthogonal direction.
Regardless of species being implanted, a significant objective in the ion implantation of semiconductor wafers is to obtain the correct cumulative ion dose, plus ion dose uniformity throughout the wafer, or portions of the wafer targeted by the implanted ions. The implanted dose determines the electrical activity of the implanted region, while dose uniformity insures that all devices on the semiconductor wafer have operating characteristics within specified limits. Semiconductor fabrication processes frequently call for dose accuracy within one percent, and dose uniformity of less than one percent.
Some implantation systems monitor dose by measuring beam current during an implant process using an ion beam current detector (usually a Faraday cup) positioned xe2x80x98behindxe2x80x99 the plane of the wafer so that, as the beam and the wafer move relative to each other, the beam can fall on the Faraday cup. When multiple wafers are being implanted, this may be achieved by positioning the cup behind the movable (usually rotatably) wafer holder with one or more gaps/slits in the holder through which the beam can pass to the cup which is aligned with the general path of the beam. The measured beam current includes electrons generated during ion implantation, and excludes neutral molecules that are implanted into the target, even though these neutral molecules contribute to the total dose. Given that the current is measured after the beam passes through the wafer being implanted, the measured current thus depends upon the characteristics of the wafer, which may produce errors in the measured current due to interactions of the beam with materials on or within the wafer. These wafer characteristics include emissivity, local charging, gas emission from photoresist on the wafer, and the like.
In conventional ion implantation systems that involve the application of a high-energy beam to the wafer, cumulative ion dose is also typically measured by a Faraday cup, or a Faraday cage, situated in the vicinity of the target wafer. The Faraday cage is typically a conductive enclosure, often with the wafer located at the downstream end of the enclosure and constituting part of the Faraday system. The ion beam passes through the Faraday cage to the wafer and produces an electrical current in the Faraday. The Faraday current is supplied to an electronic dose processor, which integrates the current with respect to time to determine the total ion dosage. The dose processor may be part of a feedback loop that is used to control the ion implanter. Dose uniformity can be monitored by a corner cup arrangement. The beam is scanned over the area of the mask with the portion passing through the central opening impinging on the target wafer. Small Faraday cups are located at the four corners of the mask and sense the beam current at these locations. Individual conductors at each corner connect the four corner cups to a monitoring system which determines the deviation of the beam current at each corner from an average value. In some systems, the corner cups have been connected in common for measurement of cumulative ion dose.
Beam ions are generally neutralized by collisions with residual gas molecules in the chamber and it is known that the proportion of ions that become neutralized increases with increasing residual gas pressure. Collisions may also result in the state of charge of beam ions being increased, e.g. from singly to doubly charged or reduced, e.g. from doubly to singly charged. Both of these effects can contribute to beam current measuring errors.
Some ion implantation equipment manufacturers attempt to compensate for neutral ions in an ion beam in the dose control system of their ion implanters by measuring the gas pressure in the implantation volume. This gas pressure signal is then used to calculate an effective or corrected beam current value in accordance with a predetermined relationship between the gas pressure, the apparent or measured beam current and a term which is commonly referred to as the Pressure Compensation factor (PCF). The resulting effective beam current value is then supplied to the dose control system. A dynamic mode of operation is utilized in which values for PCF are determined during an ion implantation process, by comparing simultaneous measurements of measured ionic current and pressure, with corresponding simultaneous measurements taken previously during the process. However, a linear relationship is assumed between corrected beam current and residual gas pressure, which provides limited accuracy, particularly at higher beam energies. An additional parameter, xcex3, interpreted as the ratio of final steady charge state to the initial injected charge state, has also been disclosed in the prior art. Both PCF and xcex3 are determined empirically prior to performing production implant runs, and the values stored for each particular process recipe to be optimized. The requirement of performing multiple test implants in order to assemble empirical values for PCF and xcex3 for each implant recipe is a time consuming, labor-intensive effort. Moreover, an unexpected parameter change during an implant process could result in the computations being inaccurate, because the computations use the pre-determined PCF and xcex3 values to calculate a corrected beam current. This inaccuracy would result in unacceptable dose errors.
Other ion implantation equipment manufacturers control the dose by compensating for vacuum fluctuations during an implantation process based on detected beam current and not based on a detected gas pressure within an implantation chamber. This is accomplished by determining a reference value for the ion beam current by measuring the ion beam current when a vacuum level along the beam line is at a relatively high and stable level before implantation of a wafer begins. Once the reference value is established, implantation of the wafer is performed, and the ion beam current is measured during implantation. A difference between the reference value and the measure ion beam current is determined, and the implantation process parameters, e.g., ion beam current, or a wafer scan rate, may be adjusted accordingly. This method compensates mainly for non-line-of-sight collisions to adjust the wafer dose accordingly.
Various Faraday cage and Faraday cup configurations (beam current detectors) for ion implanters have been disclosed in the prior art. However, the existing beam current detectors typically detect only charged particles, but not neutral particles. These neutral particles may be caused by photoresist or other materials coated on a wafer that outgas, volatize, or sputter when the ion beam impacts the wafer. This outgassing releases gas particles which may collide with ions in the ion beam and cause ions in the beam to undergo a charge change. These charge exchanging collisions can cause problems because the detectors used to determine and control the ion beam current (and also the total dose of the wafer) during implantation typically only detect charged particles. The neutral particles implanted in the wafer are the desired implantation species, may have the desired energies for implantation, and thus should be counted in the total implant dose: However, because the detectors normally do not detect ions which have been neutralized prior to being implanted in the wafer, they typically understate the true rate of delivery of desired species, including both ions and neutrals, in the beam.
Therefore, what is needed is a method for compensating for the contribution of uncharged or neutral particles to the total ion dose implanted into semiconductor wafers during an implantation process.